Transflective electrophoretic display

ABSTRACT

This invention relates to an electrophoretic display comprising isolated cells of well-defined shape, size and aspect ratio which cells are filled with charged pigment particles dispersed in a solvent, a backlight and optionally a background layer. The display may have the traditional up/down switching mode, an in-plane switching mode or a dual switching mode.

This application claims the benefit of provisional application No.60/314,272 filed Aug. 20, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a transflective electrophoretic displaycomprising isolated cells of well-defined shape, size and aspect ratio.The cells are filled with charged particles dispersed in a dielectricsolvent and the display has a backlight.

The display may have the traditional up/down switching mode, thein-plane switching mode or the dual switching mode.

2. Description Of Related Art

The electrophoretic display (EPD) is a non-emissive device based on theelectrophoresis phenomenon influencing charged pigment particlessuspended in a colored dielectric solvent. This general type of displaywas first proposed in 1969. An EPD typically comprises a pair ofopposed, spaced-apart plate-like electrodes, with spacers predetermininga certain distance between the electrodes. At least one of theelectrodes, typically on the viewing side, is transparent. For thepassive type of EPDs, row and column electrodes on the top (the viewingside) and bottom plates respectively are needed to drive the displays.In contrast, an array of thin film transistors (TFT) on the bottom plateand a common, non-patterned transparent conductor plate on the topviewing substrate are required for the active type EPDs. Anelectrophoretic fluid composed of a colored dielectric solvent andcharged pigment particles dispersed therein is enclosed between the twoelectrodes.

When a voltage difference is imposed between the two electrodes, thepigment particles migrate by attraction to the plate of polarityopposite that of the pigment particles. Thus, the color showing at thetransparent plate, determined by selectively charging the plates, can beeither the color of the solvent or the color of the pigment particles.Reversal of plate polarity will cause the particles to migrate back tothe opposite plate, thereby reversing the color. Intermediate colordensity (or shades of gray) due to intermediate pigment density at thetransparent plate may be obtained by controlling the plate chargethrough a range of voltages.

A transmissive EPD is disclosed in U.S. Pat. No. 6,184,856 in which abacklight, color filters, and substrates with two transparent electrodesare used. The electrophoretic cells serve as a light valve. In thecollected state, the particles are positioned to minimize the coverageof the horizontal area of the cell and allow the backlight to passthrough the cell. In the distributed state, the particles are positionedto cover the horizontal area of the pixel and scatter or absorb thebacklight. However, the backlight and color filter used in this deviceconsume a great deal of power and are not desirable for hand-helddevices such as PDAs (personal digital assistants) and e-books.

EPDs of different pixel or cell structures have been reported in priorart, for example, the partition-type EPD (M. A. Hopper and V. Novotny,IEEE Trans. Electr. Dev., Vol. ED 26, No. 8, pp. 1148-1152 (1979)) andthe microencapsulated EPD (U.S. Pat. Nos. 5,961,804 and 5,930,026), andeach of these has its own problems as noted below.

In a partition-type EPD, there are partitions between the two electrodesfor dividing the space into smaller cells in order to prevent undesiredmovements of the particles such as sedimentation. However, difficultiesare encountered in the formation of the partitions, the process offilling the display with the fluid, enclosing the fluid in the display,and keeping the suspensions of different colors separated from eachother.

The microencapsulated EPD has a substantially two dimensionalarrangement of microcapsules each having therein an electrophoreticcomposition of a dielectric fluid and a dispersion of charged pigmentparticles that visually contrast with the dielectric solvent. Themicrocapsules are typically prepared in an aqueous solution and, toachieve a useful contrast ratio, their mean particle size is relativelylarge (50-150 microns). The large microcapsule size results in a poorscratch resistance and a slow response time for a given voltage becausea large gap between the two opposite electrodes is required for largecapsules. Also, the hydrophilic shell of microcapsules prepared in anaqueous solution typically results in sensitivity to high moisture andtemperature conditions. If the microcapsules are embedded in a largequantity of a polymer matrix to obviate these shortcomings, the use ofthe matrix results in an even slower response time and/or a lowercontrast ratio. To improve the switching rate, a charge-controllingagent is often needed in this type of EPDs. However, themicroencapsulation process in an aqueous solution imposes a limitationon the type of charge controlling agents that can be used. Otherdrawbacks associated with the microcapsule system include poorresolution and poor addressability for color applications.

An improved EPD technology was recently disclosed in co-pendingapplications, U.S. Ser. No. 09/518,488, filed on Mar. 3, 2000(corresponding to WO01/67170), U.S. Ser. No. 09/759,212, filed on Jan.11, 2001, U.S. Ser. No. 09/606,654, filed on Jun. 28, 2000(corresponding to WO02/01280) and U.S. Ser. No. 09/784,972, filed onFeb. 15, 2001, all of which are incorporated herein by reference. Theimproved EPD comprises closed isolated cells formed from microcups ofwell-defined shape, size and aspect ratio and filled with chargedpigment particles dispersed in a dielectric solvent. The electrophoreticfluid is isolated and sealed in each microcup.

The microcup structure, in fact, enables a format flexible, efficientroll-to-roll continuous manufacturing process for the preparation ofEPDs. The displays can be prepared on a continuous web of conductor filmsuch as ITO/PET by, for example, (1) coating a radiation curablecomposition onto the ITO/PET film, (2) making the microcup structure bya microembossing or photolithographic method, (3) filling theelectrophoretic fluid and sealing the microcup, (4) laminating thesealed microcups with the other conductor film, and (5) slicing andcutting the display to a desirable size or format for assembling.

One advantage of this EPD design, is that the microcup wall is in fact abuilt-in spacer to keep the top and bottom substrates apart at a fixeddistance. The mechanical properties and structural integrity of microcupdisplays are significantly better than any prior art displays includingthose manufactured by using spacer particles. In addition, displaysinvolving microcups have desirable mechanical properties includingreliable display performance when the display is bent, rolled or undercompression pressure from, for example, a touch screen application. Theuse of the microcup technology also eliminates the need of an edge sealadhesive which would limit and predefine the size of the display paneland confine the display fluid inside a predefined area. The displayfluid within a conventional display prepared by the edge sealingadhesive method will leak out completely if the display is cut in anyway, or if a hole is drilled through the display. The damaged displaywill be no longer functional. In contrast, the display fluid within thedisplay prepared by the microcup technology is enclosed and isolated ineach cell. The microcup display may be cut to almost any dimensionwithout the risk of damaging the display performance due to the loss ofdisplay fluids in the active areas. In other words, the microcupstructure enables a format flexible display manufacturing process,wherein the process produces a continuous output of displays in a largesheet format which can be sliced and diced to any desired format. Theisolated microcup or cell structure is particularly important when cellsare filled with fluids of different specific properties such as colorsand switching rates. Without the microcup structure, it will be verydifficult to prevent the fluids in adjacent areas from intermixing orbeing subject to cross-talk during operation.

SUMMARY OF THE INVENTION

To view a reflective display, an external light source is needed. Forthe applications to be viewed in the dark, a transflective EPD equippedwith a backlight system is typically preferred over a reflective EPDwith a front pilot light because of cosmetic and light managementreasons. However, the presence of light scattering particles in the EPDcells greatly reduces the efficiency of the backlight system. A highcontrast ratio in both bright and dark environments is difficult toachieve for traditional EPDs.

The present invention is directed to a transflective EPD employing themicrocup technology. The display comprises isolated cells prepared fromthe microcups of well-defined shape, size and aspect ratio and abacklight. The cells are filled with charged pigment particles dispersedin a dielectric solvent

For applications to be viewed in dark environments, the microcupstructure effectively allows the backlight to reach the viewer throughthe microcup walls. Thus, even a low intensity backlight is sufficientfor users to view the transflective EPDs. A photocell sensor to modulatethe backlight intensity might also be used to further reduce the powerconsumption of such EPDs.

The display may have the traditional up/down switching mode, thein-plane switching mode or the dual switching mode.

In the display having the traditional up/down switching mode or the dualswitching mode, there are a top transparent electrode plate, a bottomelectrode plate and a plurality of isolated cells enclosed between thetwo electrode plates. In the display having the in-plane switching mode,the cells are sandwiched between a top transparent insulator layer and abottom electrode plate.

The isolated cells are separated by light-transmissive cell walls. Thebacklight is underneath the bottom electrode plate. The display may alsohave a background layer above the back light system to control the lightpassing through the display. The background layer when used ispreferably gray. A background layer below the backlight system may alsobe used to improve the contrast ratio.

A diffuser may also be added onto the top of the transflective displayto enhance the viewability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a side view of the electrophoretic display of the presentinvention.

FIG. 1B is a top view of the electrophoretic display of the presentinvention.

FIG. 1C is the side view of the electrophoretic display having thein-plane switching mode.

FIG. 1D is the side view of the electrophoretic display having the dualswitching mode.

FIG. 2 shows the preparation of the microcups involving imagewiseexposure of the conductor film coated with a UV curable composition toUV radiation.

FIG. 3 is a flow chart for manufacturing a black/white electrophoreticdisplay or other monochrome electrophoretic displays.

FIGS. 4a-4 h are a flow chart for manufacturing a full colorelectrophoretic display.

FIGS. 5A-5F illustrate the display having an in-plane switching mode.

FIGS. 6A-6C illustrate the display having a dual switching mode.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

Unless defined otherwise in this specification, all technical terms areused herein according to their conventional definitions as they arecommonly used and understood by those of ordinary skill in the art.

The term “microcup” refers to the cup-like indentations created bymicroembossing or imagewise exposure.

The term “cell”, in the context of the present invention, is intended tomean the single unit formed from a sealed microcup. The cells are filledwith charged pigment particles dispersed in a solvent or solventmixture.

The term “well-defined”, when describing the microcups or cells, isintended to indicate that the microcup or cell has a definite shape,size and aspect ratio which are pre-determined according to the specificparameters of the manufacturing process.

The term “aspect ratio” is a commonly known term in the art ofelectrophoretic displays. In this application, it refers to the depth towidth or depth to length ratio of the microcups.

The term “isolated” refers to the electrophoretic cells which are sealedwith a sealing layer so that the electrophoretic composition enclosedwithin one cell cannot be transferred to other cells.

Preferred Embodiments

An electrophoretic display (100) of the present invention, as shown inFIG. 1A, comprises a top transparent layer (101), one bottom electrodeplate (102) and a layer of isolated cells (103) enclosed between the twoelectrode plates. The top transparent layer (101) is a transparentconductor film such as ITO on a PET for the traditional up/downswitching mode or the dual switching mode or a transparent insulatorlayer for the in-plane switching mode.

The cells (103 a, 103 b and 103 c) are of well-defined shape, size andaspect ratio and are filled with charged particles (104) dispersed in adielectric solvent (105). The isolated cells are sealed with a sealinglayer (106). The top transparent layer is usually laminated over thesealed cells with an adhesive layer. A backlight (107) is placedunderneath the bottom electrode plate layer (102). The backlight may beon the side edge of the display and illuminate the display panel througha diffusing light pipe. The display may optionally have a backgroundlayer (108) underneath the bottom electrode plate (102) or the backlight system (107).

FIG. 1B is the top view of the cells. As shown in this figure, theperimeter of each cell is defined by partition vertical side walls(109). The side walls extend between the top transparent layer (101) andthe bottom electrode plate (102). While the side walls in FIG. 1A areshown to be perpendicular to the top transparent layer and the bottomelectrode plate, it is understood that the side walls may be tilted tofacilitate manufacturing of the cells (i.e., mold release disclosed inSection I(a) below). In the present invention, the side walls are formedfrom a light transmissive material.

In the display having the traditional up/down switching mode, the toptransparent layer (101) is an electrode plate or film.

In the display having an in-plane switching mode (FIG. 1C), the toptransparent layer (101) is an insulator substrate and the bottomelectrode plate comprises in-plane electrodes (110 a and 110 b) and abottom electrode (111) between the two in-plane electrodes separated bygaps (112). Alternatively, the bottom layer may have only one in-planeswitching electrode, and one bottom electrode with a gap in between.

In the display having a dual switching mode (FIG. 1D), the top layer(101) contains a transparent top electrode (116, not shown). The bottomelectrode plate has one in-plane electrode (113 a) on the left-handside, a bottom electrode (114) and another in-plane electrode (113 b) onthe right-hand side. There are gaps (115) separating the in-plane andbottom electrodes. Alternatively, the bottom electrode plate may haveonly one in-plane electrode and one bottom electrode with a gap inbetween (not shown).

I. Preparation of the Microcups

The microcups generally may be manufactured by microembossing orphotolithography as disclosed in U.S. Ser. No. 09/518,488 filed Mar. 3,2000 (corresponding to WO01/67170) and U.S. Ser. No. 09/784,972 filed onFeb. 15, 2001.

I(A) Preparation of the Microcups by Embossing

Preparation of the Male Mode

The male mold may be prepared by any appropriate method, such as adiamond turn process or a photoresist process followed by either etchingor electroplating after the resist is developed. A master template forthe male mold may be manufactured by any appropriate method, such aselectroplating. With electroplating, a glass base is sputtered with athin layer (typically 3000 Å) of a seed metal such as chrome inconel. Itis then coated with a layer of photoresist and exposed to radiation,such as ultraviolet (UV). A mask is placed between the UV and the layerof photoresist. The exposed areas of the photoresist become hardened.The unexposed areas are then removed by washing them with an appropriatesolvent. The remaining hardened photoresist is dried and sputtered againwith a thin layer of seed metal. The master is then ready forelectroforming. A typical material used for electroforming is nickelcobalt. Alternatively, the master can be made of nickel byelectroforming or electroless nickel deposition as described in“Continuous Manufacturing of Thin Cover Sheet Optical Media”, SPIE Proc.1663:324 (1992). The floor of the mold is typically between about 50 to400 microns thick. The master can also be made using othermicroengineering techniques including e-beam writing, dry etching,chemical etching, laser writing or laser interference as described in“Replication Techniques for Micro-optics”, SPIE Proc. 3099:76-82 (1997).Alternatively, the mold can be made by photomachining using plastics,ceramics or metals.

The male mold thus prepared typically has protrusions between about 3 to500 microns, preferably between about 5 to 100 microns, and mostpreferably about 10 to 50 microns. The male mold may be in the form of abelt, a roller or a sheet. For continuous manufacturing, the belt or theroller type of mold is preferred. Prior to applying a UV curable resincomposition, the mold may be treated with a mold release to aid in thedemolding process.

Microcup Formation

Microcups may be formed either in a batchwise process or in a continuousroll-to-roll process as disclosed in the co-pending application, U.S.Ser. No. 09/784,972, filed on Feb. 15, 2001. The latter offers acontinuous, low cost and high throughput manufacturing technology forproduction of compartments for use in electrophoretic or LCDs. Prior toapplying a UV curable resin composition, the mold may be treated with amold release to aid in the demolding process. To further improve thedemolding process, the conductor film may be precoated with a primer oran adhesion promoting layer to improve the adhesion between theconductor film and the microcups.

The UV curable resin may be degassed prior to dispensing and mayoptionally contain a solvent. The solvent, if present, readilyevaporates. The UV curable resin is dispensed by any appropriate meanssuch as, coating, dipping, pouring and the like, over the male mold. Thedispenser may be moving or stationary. For the manufacture of thedisplay having the traditional up/down switching mode or the dualswitching mode, the UV curable resin is coated onto a conductor film.Examples of suitable conductor film include transparent conductor ITO onplastic substrates such as polyethylene terephthalate, polyethylenenaphthate, polyaramid, polyimide, polycycloolefin, polysulfone andpolycarbonate. Pressure may be applied, if necessary, to ensure properbonding between the resin and the plastic and to control the thicknessof the floor of the microcups. The pressure may be applied using alaminating roller, vacuum molding, press device or any other like means.If the male mold is metallic and opaque, the plastic substrate istypically transparent to the actinic radiation used to cure the resin.Conversely, the male mold can be transparent and the plastic substratecan be opaque to the actinic radiation. To obtain good transfer of themolded features onto the transfer sheet, the conductor film needs tohave good adhesion to the UV curable resin which should have a goodrelease property against the mold surface.

For the manufacture of displays having an in-plane switching mode, atransparent insulator substrate may be used in the embossing step.Suitable transparent insulator substrates include polyethyleneterephthalate, polyethylene naphthate, polyaramid, polyimide,polycycloolefin, polysulfone and polycarbonate.

The UV curable composition used for the preparation of the microcups maycomprise polyvalent acrylate or methacrylate, polyvalent vinyl includingvinylbenzene, vinylsilane, vinylether, polyvalent epoxide, polyvalentallyl, oligomers or polymers containing crosslinkable functional groupsand the like. Multifunctional acrylate and its oligomers are the mostpreferred. A combination of a multifunctional epoxide and amultifunctional acrylate is also very useful to achieve desirablephysico-mechanical properties. A crosslinkable oligomer impartingflexibility, such as urethane acrylate or polyester acrylate, is usuallyalso added to improve the flexure resistance of the embossed microcups.The composition may contain oligomer, monomer, additives and optionallya polymer. The glass transition temperatures (or Tg) for this class ofmaterials usually range from about −70° C. to about 150° C., preferablyfrom about −20° C. to about 50° C. The microembossing process istypically carried out at a temperature higher than the Tg. A heated malemold or a heated housing substrate against which the mold presses may beused to control the microembossing temperature and pressure. The malemold may be released from the microcups formed during or after the UVcurable resin is hardened by the UV.

I(b) Preparation of the Microcups by Photolithographic Process

Alternatively, the microcups for the displays may be prepared by aphotolithographic process. FIG. 2 illustrates the preparation ofmicrocups by imagewise exposure.

As shown in FIGS. 2A and 2B, the microcup array (20) may be prepared byexposure of a radiation curable material (21 a), coated by any knownmethods onto a transparent patterned conductor film (22), to UV light(or alternatively other forms of radiation, electron beams and the like)through a mask (26) to form walls (21 b) corresponding to the imageprojected through the mask (26). The conductor film (22) is on a plasticsubstrate (23).

In the photomask (26) in FIG. 2A, the dark squares (24) represent thearea opaque to the radiation employed, and the space (25) between thedark squares represents the radiation-transparent area. The UV radiatesthrough the opening area (25) onto the radiation curable material (21a).

As shown in FIG. 2B, the exposed areas (21 b) become hardened and theunexposed areas protected masked by the opaque area (24) of the mask(26)) are then removed by an appropriate solvent or developer to formthe microcups (27). The solvent or developer is selected from thosecommonly used for dissolving or dispersing radiation curable materialssuch as methylethylketone toluene, acetone, ethyl acetate, isopropanolor the like. Although the microcup pattern (24) of the photomask (26) isin registration with, and aligned well to, the patterned conductor film(22) in FIG. 2A, it is generally not required particularly for lowresolution applications. Similarly, the width of the conductor lines maynot be the same as the width of the microcups.

Alternatively, the exposure can be done by placing the photomaskunderneath the conductor film/substrate. In this case, the conductorfilm/substrate must be transparent to the radiation wavelength used forexposure.

For the manufacture of displays having an in-plane switching mode, atransparent insulator substrate may be used to replace the conductorfilm. Suitable transparent insulator substrates include polyethyleneterephthalate, polyethylene naphthate, polyaramid, polyimide,polycycloolefin, polysulfone, polycarbonate and the like.

In general, the microcups can be of any shape, and their sizes andshapes may vary. The microcups may be of substantially uniform size andshape in one system. However, in order to maximize the optical effect,microcups having a mixture of different shapes and sizes may beproduced. For example, microcups filled with a dispersion of the redcolor may have a different shape or size from the green microcups or theblue microcups. Furthermore, a pixel may consist of different numbers ofmicrocups of different colors. For example, a pixel may consist of anumber of small green microcups, a number of large red microcups, and anumber of small blue microcups. It is not necessary to have the sameshape and number for the three colors.

The openings of the microcups may be round, square, rectangular,hexagonal or any other shape. The partition area between the openings ispreferably kept small in order to achieve a high color saturation andcontrast while maintaining desirable mechanical properties. Consequentlythe honeycomb-shaped opening is preferred over, for example, thecircular opening.

The dimension of each individual microcup may be in the range of about10² to about 1×10⁶ μm², preferably from about 10³ to about 1×10⁵ μm².The depth of the microcups is in the range of about 5 to about 200microns, preferably from about 10 to about 100 microns. The opening tothe total area ratio is in the range of from about 0.05 to about 0.95,preferably from about 0.4 to about 0.9.

II. Preparation of the Suspension/Dispersion

The suspensions filled in the microcups comprise a dielectric solventwith charged pigment particles dispersed therein and the particlesmigrate under the influence of an electric field. The suspensions mayoptionally contain additional colorants which do not migrate in theelectric field. The dispersion may be prepared according to methods wellknown in the art, such as U.S. Pat. Nos. 6,017,584, No. 5,914,806, No.5,573,711, No 5,403,518, No. 5,380,362, No. 4,680,103, No. 4,285,801,No. 4,093,534, No. 4,071,430, and No. 3,668,106, and as described inIEEE Trans. Electron Devices, ED-24, 827 (1977), and J. Appl. Phys.49(9), 4820 (1978).

The suspending fluid medium is a dielectric solvent which preferably hasa low viscosity and a dielectric constant in the range of about 2 toabout 30, preferably about 2 to about 15 for high particle mobility.Examples of suitable dielectric solvents include hydrocarbons such asdecahydronaphthalene (DECALIN), 5-ethylidene-2-norbornene, fatty oils,paraffin oil, aromatic hydrocarbons such as toluene, xylene,phenylxylylethane, dodecylbenzene and alkylnaphthalene, halogenatedsolvents such as dichlorobenzotrifluoride,3,4,5-trichlorobenzotrifluoride, chloropentafluoro-benzene,dichlorononane, pentachlorobenzene, and perfluoro solvents such asperfluorodecalin, perfluorotoluene, perfluoroxylene, FC-43, FC-70 andFC-5060 from 3M Company, St. Paul Minn., low molecular weight fluorinecontaining polymers such as poly(perfluoropropylene oxide) from TCIAmerica, Portland, Oreg., poly(chlorotrifluoroethylene) such asHalocarbon Oils from Halocarbon Product Corp., River Edge, N.J.,perfluoropolyalkylether such as Galden, HT-200, and Fluorolink fromAusimont or Krytox Oils and Greases K-Fluid Series from DuPont, Del. Inone preferred embodiment, poly(chlorotrifluoroethylene) is used as thedielectric solvent. In another preferred embodiment,poly(perfluoropropylene oxide) is used as the dielectric solvent.

The contrasting colorant may be dyes or pigments. Nonionic azo andanthraquinone dyes are particularly useful. Examples of useful dyesinclude, but are not limited to: Oil Red EGN, Sudan Red, Sudan Blue, OilBlue, Macrolex Blue, Solvent Blue 35, Pylam Spirit Black and Fast SpiritBlack (Pylam Products Co., Arizona), Thermoplastic Black X-70 (BASF),anthraquinone blue, anthraquinone yellow 114, anthraquinone reds 111 and135, anthraquinone green 28 and Sudan Black B (Aldrich). Fluorinateddyes are particularly useful when perfluorinated solvents are used. Inthe case of a contrasting color pigment, the color pigment particles mayalso be dispersed in the dielectric medium and these colored particlesare preferably uncharged. If the contrasting color pigment particles arecharged, they preferably carry a charge which is opposite from that ofthe charged primary pigment particles. If both of the primary andcontrasting color pigment particles carry the same charge, then theyshould have different charge density or different electrophoreticmobility. The dye or pigment used in EPDs must be chemically stable andcompatible with other components in the suspension.

The charged primary pigment particles are preferably white, and may beorganic or inorganic pigments, such as TiO₂.

If colored pigment particles are used, they may be formed fromphthalocyanine blue, phthalocyanine green, diarylide yellow, diarylideAAOT yellow, and quinacridone, azo, rhodamine, perylene pigment series(Sun Chemical), Hansa yellow G particles (Kanto Chemical), and CarbonLampblack (Fisher). Particle size is preferably in the range of 0.01-5microns, and is even more preferably in the range of 0.05-2 microns.These particles should have acceptable optical characteristics, shouldnot be swollen or softened by the dielectric solvent, and should bechemically stable. The resulting suspension must also be stable againstsedimentation, creaming or flocculation under normal operatingconditions.

The pigment particles may exhibit a native charge, or may be chargedexplicitly using a charge control agent, or may acquire a charge whensuspended in the dielectric solvent. Suitable charge control agents arewell known in the art; they may be polymeric or non-polymeric in nature,and may also be ionic or non-ionic, including ionic surfactants such asAerosol OT, sodium dodecylbenzenesulfonate, metal soaps, polybutenesuccinimide, maleic anhydride copolymers, vinylpyridine copolymers,vinylpyrrolidone copolymers (such as Ganex, International SpecialtyProducts), (meth)acrylic acid copolymers and N,N-dimethylaminoethyl(meth)acrylate copolymers. Fluorosurfactants are particularly useful ascharge controlling agents in perfluorocarbon solvents. These include FCfluorosurfactants such as FC-170C, FC-171, FC-176, FC430, FC431 andFC-740 from 3M Company and Zonyl fluorosurfactants such as Zonyl FSA,FSE, FSN, FSN-100, FSO, FSO-100, FSD and UR from Dupont.

Suitable charged pigment dispersions may be manufactured by any of thewell-known methods including grinding, milling, attriting,microfluidizing, and ultrasonic techniques. For example, pigmentparticles in the form of a fine powder are added to the suspendingsolvent and the resulting mixture is ball milled or attrited for severalhours to break up the highly agglomerated dry pigment powder intoprimary particles. Although less preferred, a dye or pigment forproducing the non-migrating fluid colorant may be added to thesuspension during the ball milling process.

Sedimentation or creaming of the pigment particles may be eliminated bymicroencapsulating the particles with suitable polymers to match thespecific gravity to that of the dielectric solvent. Microencapsulationof the pigment particles may be accomplished chemically or physically.Typical microencapsulation processes include interfacial polymerization,in-situ polymerization, phase separation, coacervation, electrostaticcoating, spray drying, fluidized bed coating and solvent evaporation.

III. Filling and Sealing of the Microcups

The filling and sealing procedures are described in the co-pending USapplications U.S. Ser. No. 09/518,488 (corresponding to WO01/67170) andU.S. Ser. No. 09/784,972 (corresponding to WO02/01280) referred to inthe previous sections, the disclosures of which are incorporated hereinby reference in their entirety.

The sealing of the microcups may be accomplished in a number of ways. Apreferred approach is to disperse a UV curable composition containingmultifunctional acrylates, acrylated oligomers and photoinitiators intoan electrophoretic fluid containing charged pigment particles dispersedin a colored dielectric solvent. The UV curable composition isimmiscible with the dielectric solvent and has a specific gravity lowerthan that of the dielectric solvent and the pigment particles. The twocomponents, UV curable composition and the electrophoretic fluid, arethoroughly blended in an in-line mixer and immediately coated onto themicrocups with a precision coating mechanism such as Myrad bar, gravure,doctor blade, slot coating or slit coating. Excess fluid is removed by awiper blade or a similar device. A small amount of a weak solvent orsolvent mixture such as isopropanol, methanol or their aqueous solutionsmay be used to clean the residual electrophoretic fluid on the topsurface of the partition walls of the microcups. Volatile organicsolvents may be used to control the viscosity and coverage of theelectrophoretic fluid. The thus-filled microcups are then dried and theUV curable composition floats to the top of the electrophoretic fluid.The microcups may be sealed by curing the supernatant UV curable layerduring or after it floats to the top. UV or other forms of radiationsuch as visible light, IR and electron beam may be used to cure and sealthe microcups. Alternatively, heat or moisture may also be employed tocure and seal the microcups, if appropriate, heat or moisture curablecompositions may be used.

A preferred group of dielectric solvents exhibiting desirable densityand solubility discrimination against acrylate monomers and oligomersare halogenated hydrocarbons and their derivatives. Surfactants may beused to improve the adhesion and wetting at the interface between theelectrophoretic fluid and the sealing materials. Useful surfactantsinclude the FC surfactants from 3M Company, Zonyl fluorosurfactants fromDuPont, fluoroacrylates, fluoromethacrylates, fluoro-substituted longchain alcohols, perfluoro-substituted long chain carboxylic acids andtheir derivatives.

Alternatively, the electrophoretic fluid and the sealing precursor maybe coated sequentially into the microcups, particularly when the sealingprecursor is at least partially compatible with the dielectric solvent.Thus, the sealing of the microcups may be accomplished by overcoating athin layer of thermoset precursor which is hardenable by radiation,heat, solvent evaporation, moisture or interfacial reactions on thesurface of the filled microcups.

Volatile organic solvents may be used to adjust the viscosity and thethickness of the coatings. When a volatile solvent is used in theovercoat, it is preferred that it is immiscible with the dielectricsolvent. In the copending patent application, U.S. Ser. No. 09/874,391,filed Jun. 4, 2001, thermoplastic elastomers have been disclosed as thepreferred sealing material. Additives such as silica particles andsurfactants may be used to improve the film integrity and coatingquality.

Interfacial polymerization followed by UV curing is very beneficial tothe sealing process. Intermixing between the electrophoretic layer andthe overcoat is significantly suppressed by the formation of a thinbarrier layer at the interface by interfacial polymerization. Thesealing is then completed by a post curing step, preferably by UVradiation. To further reduce the degree of intermixing, it is highlydesirable that the specific gravity of the overcoating is significantlylower than that of the electrophoretic fluid. The two-step overcoatingprocess is particularly useful when the dye used is at least partiallysoluble in the sealing material.

IV. Preparation of Monochrome Electrophoretic Displays

The process is illustrated by the flow diagram as shown in FIG. 3. Allmicrocups are filled with a suspension of the same color composition.The process can be a continuous roll-to-roll process comprising thefollowing steps:

1. Coat a layer of UV curable composition (30), optionally with asolvent, on a continuous web (31). The solvent, if present, readilyevaporates. Depending on applications and the display switching mode,the continuous web (31) may be a plastic substrate, a patterned ornon-patterned conductor film on a plastic substrate.

2. Emboss the UV curable composition (30) at a temperature higher thanits glass transition temperature by a pre-patterned male mold (32).

3. Release the mold from the UV curable layer (30) preferably during orafter it is hardened by the UV exposure.

4. Fill in the thus-formed array of microcups (33) with a chargedpigment dispersion (34) in a colored dielectric solvent.

5. Seal the microcups by the methods described in copending patentapplications, U.S. Ser. No. 09/518,488, filed on Mar. 3, 2000, U.S. Ser.No. 09/759,212, filed on Jan. 11, 2001, U.S. Ser. No. 09/606,654, filedon Jun. 28, 2000, U.S. Ser. No. 09/784,972, filed on Feb. 15, 2001, andU.S. Ser. No. 09/874,391, filed Jun. 4, 2001.

The sealing methods include adding to the dielectric solvent at least athermoset precursor which is incompatible with the solvent and has alower specific gravity than the solvent and the pigment particles,followed by curing the thermoset precursor optionally by radiation suchas UV or by heat or moisture during or after the thermoset precursorseparates. Alternatively, the sealing of the microcups may beaccomplished by directly overcoating and hardening the sealingcomposition over the surface of the electrophoretic fluid.

In copending patent application, U.S. Ser. No. 09/874,391, thermoplasticelastomers have been disclosed as the preferred sealing material.Examples of useful thermoplastic elastomers include ABA, and (AB)n typeof di-block, tri-block, and multi-block copolymers wherein A is styrene,α-methylstyrene, ethylene, propylene or norbonene; B is butadiene,isoprene, ethylene, propylene, butylene, dimethylsiloxane or propylenesulfide; and A and B cannot be the same in the formula. The number, n,is ≧1, preferably 1-10. Particularly useful are di-block or tri-blockcopolymers of styrene or α-methylstyrene such as SB(poly(styrene-b-butadiene)), SBS(poly(styrene-b-butadiene-b-styrene)),SIS(poly(styrene-b-isoprene-b-styrene)),SEBS(poly(styrene-b-ethylene/butylenes-b-styrene))poly(styrene-b-dimethylsiloxane-b-styrene),poly((α-methylstyrene-b-isoprene),poly(α-methylstyrene-b-isoprene-b-α-methylstyrene),poly(α-methylstyrene-b-propylenesulfide-b-α-methylstyrene),poly(α-methylstyrene-b-dimethylsiloxane-b-α-methylstyrene). Additivessuch as silica particles and surfactants may be used to improve the filmintegrity and coating quality.

6. Laminate the sealed array of electrophoretic cells with anothercontinuous web (36) comprising electrodes or conductor lines pre-coatedwith an adhesive layer (37) which may be a pressure sensitive adhesive,a hot melt adhesive, a heat, moisture or radiation curable adhesive.

The laminate adhesive may be post cured by, for example, heat or UV (38)through either side of the web. The finished product may be cut (39)after the lamination step. Alternatively, the sealed microcups may becut to appropriate size before the lamination step.

The preparation of the microcups described above can be convenientlyreplaced by the alternative procedure of imagewise exposing theconductor film coated with a thermoset precursor followed by removingthe unexposed areas by an appropriate solvent.

For the manufacture of displays having the in-plane switching mode, thethermoplastic or thermoset precursor may be coated on a transparentinsulator substrate instead of a conductor film, before microembossingor imagewise exposure.

V. Preparation of Multi-Color Electrophoretic Displays

The methods described in copending patent applications, U.S. Ser. No.09/518,488, filed on Mar. 3, 2000 and U.S. Ser. No. 09/879,408, filed onJun. 11, 2001 may be used to prepare sealed microcups containingelectrophoretic fluids of different colors. The process includes (1)laminating the already formed microcups with a positively workingdry-film photoresist consisting of at least a removable support such asPET-4851 from Saint-Gobain, Worcester, Mass., a novolac positivephotoresist such as Microposit S1818 from Shipley, and analkali-developable adhesive layer such as a mixture of Nacor 72-8685from National Starch and Carboset 515 from BF Goodrich; (2) selectivelyopening a certain amount of the microcups by imagewise exposing thephotoresist, removing the removable support film, and developing thepositive photoresist with a developer such as diluted Microposit 351developer from Shipley; (3) filling the opened cups with theelectrophoretic fluid containing charged white pigment (TiO₂) particlesand dye or pigment of the first primary color; and (4) sealing thefilled microcups as described in the preparation of monochrome displays.These additional steps may be repeated to create microcups filled withelectrophoretic fluid of the second and the third primary colors.

More specifically, a multi-color electrophoretic displays may beprepared according to the steps as shown in FIG. 4:

1. Coat a layer of thermoset precursor (40) on a conductor film (41).

2. Emboss the thermoplastic or thermoset precursor layer at atemperature higher than its glass transition temperature by apre-patterned male mold (not shown).

3. Release the mold from the thermoplastic or thermoset precursor layerpreferably during or after it is hardened by cooling or crosslinking byradiation, heat or moisture.

4. Laminate the thus formed array of microcups (42) with a positivedry-film photoresist which comprises at least an adhesive layer (43), apositive photoresist (44) and a removable plastic cover sheet (notshown).

5. Imagewise expose (FIG. 4c) the positive photoresist by UV, visiblelight, or other radiation, remove the cover sheet, develop and open cupsin the exposed area. The purpose of Steps 4 and 5 is to selectively openthe microcups in a predetermined area (FIG. 4d).

6. Fill in the opened microcups with a charged white pigment dispersion(45) in a dielectric solvent containing at least a dye or pigment of thefirst primary color and a thermoset precursor (46) which is incompatiblewith the solvent and has a lower specific gravity than the solvent andthe pigment particles.

7. Seal the microcups to form closed electrophoretic cells containingelectrophoretic fluid of the first primary color by curing the thermosetprecursor (preferably by radiation such as UV, less preferably by heator moisture) during or after the thermoset precursor separates and formsa supernatant layer on top of the liquid phase (FIG. 4e).

8. Steps 5-7 described above may be repeated to generate well-definedcells containing electrophoretic fluids of different colors in differentareas (FIGS. 4e, 4 f and 4 g).

9. Laminate the sealed array of electrophoretic cells in registration toa second, pre-patterned transparent conductor film (47) pre-coated withan adhesive layer (48) which may be a pressure sensitive adhesive, a hotmelt adhesive, a heat, moisture or radiation curable adhesive.

10. Harden the adhesive.

In the Step 4 above, lamination of a dry film positively workingphotoresist onto the microcups may be replaced by directly coating thepositive photoresist onto the microcups. Removable fillers may be usedto fill the microcups before the photoresist is coated or laminated ontothe microcups. In this case, no cover sheet is needed. This is disclosedin copending application U.S. Ser. No. 09/879,408, filed on Jun. 11,2001.

Suitable materials for use as a filler include inorganic, organic,organometallic and polymeric materials, or their particulates. Morepreferable filler materials are non-film forming particles such aslatexes of PMMA, polystyrene, polyethylene and their carboxylatedcopolymers and their corresponding salts, wax emulsions, colloidalsilica, titanium oxide, and calcium carbonate dispersions or mixturesthereof. Particularly preferable filler materials include aqueousdispersions of ionomers of ethylene copolymers such as ACqua220,ACqua240, and ACqua250 (Honeywell, N.J.).

The multi-color displays having the in-plane switching mode may besimilarly prepared except that the layer of thermoset precursor in Step1 may be coated on a transparent insulator substrate instead of theconductor film.

The preparation of the microcups described in the process above canconveniently be replaced by the alternative procedure of imagewiseexposing the conductor film coated with a thermoset precursor followedby removing the unexposed areas by an appropriate solvent.

The sealing of the microcups may be alternatively accomplished bydirectly coating a layer of the thermoset precursor material over thesurface of the liquid phase.

Alternatively, color EPDs of this invention may be achieved by usingcolor filters on the top of the displays as disclosed in a copendingU.S. Ser. No. 60/308,437, filed on Jul. 27, 2001 or using colorbackground at the bottom of the displays as disclosed in anothercopending application, U.S. Ser. No. 60/306,312, filed on Jul. 17, 2001.

The thickness of the display produced by the present processes asdescribed can be as thin as a piece of paper. The width of the displayis the width of the coating web (typically 1-90 inches). The length ofthe display can be anywhere from inches to thousands of feet dependingon the size of the roll.

The optional background layer may be added by painting, printing,coating or laminating a color layer to the bottom of displays. Toincrease contrast ratio, a black or gray background layer is mostpreferred.

An active matrix EPD may also be prepared by using thin film transistors(TFT) on the bottom electrode plate of the display. The backlight systemmay be placed on the side edge of the display and is underneath thebackground color layer or between the display and the background layer.A diffusing light pipe such as filled polymethylmethacrylate may also beused to enhance the performance of the backlight.

VI. Displays of the Present Invention

Three types of switching mechanism are described in this section. Toview a reflective EPD, an external light source is needed in all cases.For the applications to be viewed in the dark, either a backlight systemor a front pilot light system may be used. A transflective EPD equippedwith a backlight system is typically preferred to a reflective EPD witha front pilot light because of cosmetic and light management reasons.However, the presence of light scattering particles in the EPD cellsgreatly reduces the efficiency of the backlight system. High contrastratio in both bright and dark environments is difficult to achieve fortraditional EPDs.

In contrast, the displays of the present invention based on the microcuptechnology effectively allows the backlight to reach the viewer throughthe walls of the microcups. Thus, even a low intensity backlight issufficient for users to view the transflective EPDs. A photocell sensorto modulate the backlight intensity might also be used to further reducethe power consumption of such EPDs.

VI(a) Display With Up/Down Switching ModeE

When there is a voltage difference between the top and bottom electrodeplates, the charged particles migrate to the top or bottom of the cells.When the particles migrate to and remain at the top of the cells, thecolor of the particles is seen through the top transparent layer. Whenthe particles migrate to and remain at the bottom of the cells, thecolor of the dielectric solvent is seen through the top transparentlayer.

VI(b) Display With In-Plane Switching Mode

For a monochrome display, in the cell as shown in FIG. 5A, whiteparticles are dispersed in a clear, colorless dielectric solvent. Thebackground of all cells is of the same color (black, blue, cyan, red,magenta, etc.). When there is a voltage difference between the bottom(not shown) and the two in-plane switching electrodes (not shown), thewhite particles migrate to the sides of the cells, resulting in thecolor of the background being seen through the top transparent opening.When there is no voltage difference between the bottom and the twoin-plane electrodes, the white particles are distributed in thedielectric solvent and as a result, the color of the particles (i.e.,white) is seen through the top transparent insulator layer.

Alternatively, as shown in FIG. 5B, particles of the same color aredispersed in a clear, colorless dielectric solvent in all cells and thebackground of the cells is white. When there is a voltage differencebetween the bottom (not shown) and the two in-plane switching electrodes(not shown), the colored particles migrate to the sides of the cells,resulting in the color of the background (i.e., white) being seenthrough the top transparent opening. When there is no voltage differencebetween the two in-plane electrodes and the bottom electrode, thecolored particles are distributed in the dielectric solvent and as aresult, the color of the particles is seen through the top transparentlayer.

FIGS. 5C-5F illustrate the multiple color display having the in-planeswitching mode.

In FIG. 5C, the cells are filled with a colorless dielectric solventwith white charged particles dispersed therein, and have differentbackground colors (i.e., red, green or blue). When there is a voltagedifference between the in-plane electrodes and the bottom electrode (notshown), the white particles migrate to the sides of the cell, the colorof the background (i.e., red, green or blue) is seen from the toptransparent opening. When there is no voltage difference between thein-plane electrodes and the bottom electrode, the particles aredistributed in the dielectric solvent resulting in the white color(i.e., the color of the particles) being seen from the top transparentopening.

In FIG. 5D, the cells are filled with a colorless dielectric solventwith black particles dispersed therein, and have different backgroundcolors (i.e., red, green or blue). When there is a voltage differencebetween the in-plane electrodes and the bottom electrode (not shown),the particles migrate to the sides of the cell, the color of thebackground (i.e., red, green or blue) is seen from the top transparentopening. When there is no voltage difference between the in-planeelectrodes and the bottom electrode, the particles are distributed inthe dielectric solvent, resulting in the black color (i.e., the color ofthe particles) being seen from the top transparent opening.

FIG. 5E shows the cells filled with a colorless dielectric solvent withparticles of different colors (i.e., red, green or blue) dispersedtherein. The background of the cells is black. When there is a voltagedifference between the in-plane electrodes and the bottom electrode (notshown), the colored charged particles migrate to the sides of the cell,the color of the background (i.e., black) is seen from the toptransparent opening. When there is no voltage difference between thein-plane electrodes and the bottom electrode, the colored particles aredistributed in the dielectric solvent, resulting in the color of theparticles (i.e., red, green or blue) being seen from the top transparentopening. In this design, the black state is of high quality.

In FIG. 5F, the cells are filled with a colorless dielectric solventwith particles of different colors (red, green or blue) dispersedtherein. The background of the cells is white. When there is a voltagedifference between the in-plane electrodes (not shown) and the bottomelectrode, the particles migrate to the sides of the cell, the color ofthe background (i.e., white) is seen from the top transparent opening,resulting in a high quality white state. When there is no voltagedifference between the in-plane electrodes and the bottom electrode, theparticles are distributed in the dielectric solvent, resulting in thecolor of the particles (i.e., red, green or blue) being seen from thetop transparent opening.

As shown in these figures, the in-plane switching mode allows theparticles to move in the planar (left/right) direction, and differentcolor combinations of particles, background, and fluid, wherein each isindividually white, black, red, green or blue, can be used to generatevarious multi-color EPDs.

In addition, the particles in the dielectric solvent may be of mixedcolors and the cells have the same background color.

The top transparent viewing layer of the display may be colored or byadding a color filter. In this case, the cells are filled with anelectrophoretic composition comprising white charged particles in aclear colorless or colored dielectric solvent and the background of thecells may be black. In a monochrome display, the transparent viewinglayer on each pixel is of the same color (such as black, red, green,blue, yellow, cyan, magenta, etc.). In a multiple color display, thetransparent viewing layers may be of different colors.

VI(c) Display With Dual Switching Mode

For the purpose of illustration, it is assumed that the white particleswith positive charge are applied throughout this application. As shownin FIGS. 6A-6C, the dual switching mode allows the particles to move ineither the vertical (up/down) direction or the planar (left/right)direction. For example, in FIG. 6A, the voltage of the top electrode isset low, and the voltages of the bottom electrode and the in-planeelectrodes are set high. The white particles migrate to and gather atthe top transparent conductor film, and the white color (i.e., the colorof the particles) is seen by the viewer.

In FIG. 6B, the in-plane electrodes are set at low voltages, and the topand the bottom electrodes are set at high voltages. In this scenario,the white particles migrate to the sides of the cells, the color seenthrough the top transparent conductor film therefore is the color of thebackground (i.e., black).

In FIG. 6C, when the voltage of the top electrode is set high, thevoltage of the bottom electrode is set low, and the in-plane electrodesare set at a low voltage, the white particles migrate to the bottom ofthe cells. In this scenario, the color of the fluid (i.e., red, green orblue) is seen by the viewer through the top transparent conductor filmas shown in the red cell of FIG. 6C. To present a red pixel in a fullcolor display, the white particles in the green and blue cells may beeither attracted to the sides as shown in FIG. 6C or to the top (notshown). The former is preferred because it typically exhibits a bettercolor saturation than the latter. Thus the dual switching modetechnology gives a full-color EPD wherein all colors including red,green, blue, black and white of high quality are available in the samedevice.

Furthermore, the background color may be of any color (e.g., cyan,yellow or magenta) instead of the commonly used black color. Forexample, the cells may be filled with a red clear dielectric solventwith white positively charged particles dispersed therein and thebackground color of the cells may be yellow. In this case, when theparticles migrate to the top, the white color (i.e., the color of theparticles) is seen by the viewer and when the particles migrate to coverthe bottom of the cells, the color of the medium (i.e., red) is seenthrough the transparent conductor. However, when the white particlesmigrate to the sides of the cells, the color seen through the toptransparent conductor film, will be a shade of orange.

Other shades or color tones may be achieved by using differentparticle/medium/background color combinations, for example,white/red/cyan, white/red/magenta, white/blue/yellow, white/blue/cyan,white/blue/magenta, white/green/yellow, white/green/cyan,white/blue/magenta, etc.

The preferred combination to achieve a full color display is whiteparticles, black background, and fluids separately colored with theadditive primary colors (i.e., red, green or blue).

A further aspect of the invention is a monochrome display with highlightoptions. In such a case, all cells in the display have the samebackground color and are filled with the same electrophoretic fluid(i.e., having the same particle/solvent color combination). For example,the display may have white particles, the solvent is one of the primarycolors (red, green or blue) and the background color is a colorcontrasting the solvent color. This arrangement is useful for arelatively simple two color device with a colored highlight option. Forexample, an EPD having white particles, a yellow dielectric solvent, anda black background can display at least three different colors in eachpixel. When the white particles are all attracted to the top viewing rowelectrode, the pixel is seen white. When the white particles areuniformly attracted to the bottom column electrodes, the pixel is seenyellow. When the white particles are attracted to the in-plane electrodeon either side of the cell, the pixel is seen black. Intermediate colorsare also possible if the particles are driven to intermediate states.

While the present invention has been described with reference to thespecific embodiments thereof, it should be understood by those skilledin the art that various changes may be made and equivalents may besubstituted without departing from the true spirit and scope of theinvention. In addition, many modifications may be made to adapt aparticular situation, materials, compositions, processes, process stepor steps, to the objective, spirit and scope of the present invention.All such modifications are intended to be within the scope of the claimsappended hereto.

What is claimed is:
 1. An electrophoretic display comprising (a) a toptransparent electrode plate; (b) a plurality of cells, said cells (i)are filled with charged pigment particles dispersed in a dielectricsolvent or solvent mixture, (ii) have a background and (iii) areseparated by light transmissive side walls; (c) a bottom electrodeplate; and (d) a backlight underneath the bottom electrode plate; andsaid electrophoretic display has both up/down and in-plane switchingmodes.
 2. The electrophoretic display of claim 1 further comprising abackground layer above or below said backlight.
 3. The electrophoreticdisplay of claim 1 wherein the plurality of cells comprises cells ofdifferent sizes and shapes.
 4. The electrophoretic display of claim 1wherein the cells are non-spherical.
 5. The electrophoretic display ofclaim 1 wherein the cells are formed from microcups with an opening arearanging from about 10² to about 1×10⁶ μm².
 6. The electrophoreticdisplay of claim 1 wherein the cells are formed from microcups with anopening area ranging from about 10³ to about 1×10⁵ μm².
 7. Theelectrophoretic display of claim 1 wherein the cells are formed frommicrocups with an opening having a circular, polygonal, hexagonal,rectangular or square shape.
 8. The electrophoretic display of claim 1wherein the cells have a depth in the range from about 5 to about 200microns.
 9. The electrophoretic display of claim 1 wherein the cellshave a depth in the range from about 10 to about 100 microns.
 10. Theelectrophoretic display of claim 1 wherein the cells have an opening tototal area ratio in the range from about 0.05 to about 0.95.
 11. Theelectrophoretic display of claim 1 wherein the cells have an opening tototal area ratio in the range from about 0.4 to about 0.9.
 12. Theelectrophoretic display of claim 1 wherein all of said charged pigmentparticles are white.
 13. The electrophoretic display of claim 1 whereinall of said charged pigment particles are of a color selected from thegroup consisting of red, green, blue, yellow, cyan and magenta.
 14. Theelectrophoretic display of claim 12 wherein said dielectric solvent isof a color selected from the group consisting of red, green, blue,yellow, cyan and magenta.
 15. The electrophoretic display of claim 12wherein said background is of a color selected from the group consistingof black, red, green, blue, yellow, cyan and magenta.
 16. Theelectrophoretic display of claim 2 wherein said bottom electrode plateor said background layer is of a color selected from the groupconsisting of black, red, green, blue, yellow, cyan and magenta.
 17. Theelectrophoretic display of claim 1 wherein said electrophoretic displayis a monochrome display.
 18. The electrophoretic display of claim 17wherein all of said cells have white charged pigment particles.
 19. Theelectrophoretic display of claim 17 wherein all of said cells havecharged pigment particles of the same color selected from the groupconsisting of red, green, blue, yellow, cyan and magenta.
 20. Theelectrophoretic display of claim 17 wherein all of said cells have adielectric solvent of the same color selected from the group consistingof red, green, blue, yellow, cyan and magenta.
 21. The electrophoreticdisplay of claim 15 wherein all of said cells have a black background.22. The electrophoretic display of claim 17 wherein all of said cellshave the background of the same color selected from the group consistingof red, green, blue, cyan, yellow and magenta.
 23. The electrophoreticdisplay of claim 1 wherein said electrophoretic display is a multi-colordisplay.
 24. The electrophoretic display of claim 23 wherein said cellsare filled with white charged pigment particles dispersed in dielectricsolvents of different colors.
 25. The electrophoretic display of claim24 wherein said colors are selected from the group consisting of red,green, blue, yellow, cyan and magenta.
 26. The electrophoretic displayof claim 25 wherein said color is red, green or blue.
 27. Theelectrophoretic display of claim 23 wherein said background is black.28. The electrophoretic display of claim 23 wherein said cells havecharged pigment particles of mixed colors and the background of the samecolor.
 29. The electrophoretic display of claim 28 wherein the mixedcolors are two or more colors selected from the group consisting ofwhite, red, green, blue, yellow, cyan and magenta.
 30. Theelectrophoretic display of claim 28 wherein said background color isselected from the group consisting of black, red, green, blue, yellow,cyan and magenta.
 31. The electrophoretic display of claim 1 whereinsaid top transparent electrode plate is colored or with a color filter.32. The electrophoretic display of claim 31 wherein all of said cellshave white charged pigment particles dispersed in a colorless dielectricsolvent.
 33. The electrophoretic display of claim 1 wherein said bottomelectrode plate comprises thin film transistors.
 34. The electrophoreticdisplay of claim 1 further comprising a diffuser on top of the display.35. The electrophoretic display of claim 1 wherein all of said cells aresealed with a sealing layer.
 36. The electrophoretic display of claim 35wherein said sealing layer is formed from a composition comprising amaterial having a specific gravity lower than that of said chargedpigment particles and that of said dielectric solvent or solventmixture.
 37. The electrophoretic display of claim 36 wherein saidmaterial is a UV curable material.
 38. The electrophoretic display ofclaim 36 wherein said material is a thermoplastic elastomer.
 39. Theelectrophoretic display of claim 36 wherein said material is selectedfrom the group consisting of polyvalent acrylates, polyvalentmethacrylates, polyvalent cyanoacrylates, polyvalent vinyls, polyvalentepoxides, polyvalent isocyanates, polyvalent allyls and oligomers orpolymers containing crosslinkable functional groups.
 40. Theelectrophoretic display of claim 1 wherein said pigment particles ineach of said cells carry the same charge and are of the same color. 41.The electrophoretic display of claim 40 wherein all of said pigmentparticles are positively charged.
 42. The electrophoretic display ofclaim 40 wherein all of said charged pigment particles are white. 43.The electrophoretic display of claim 40 wherein said dielectric solventis of a color selected from the group consisting of red, green, blue,yellow, cyan and magenta.
 44. The electrophoretic display of claim 40wherein said cells have dielectric solvents of different colors.
 45. Theelectrophoretic display of claim 40 wherein said background is black.46. The electrophoretic display of claim 40 has a tri-color system basedon the color of the charged pigment particles, color of the dielectricsolvent and color of the background.